It has been known to coat substrates such as wall-board with heavy coatings of air-curable, alkali-metal-containing silicas blended with copolymer dispersions. As taught in U.S. Pat. No. 4,002,590, such compositions are usefully heavily loaded with aggregates so as to form the heavy coatings.
It has also been well known to protect metal surfaces from corrosion by surface treatment with corrosion resistant coating materials. The corrosion resistance of silicate coatings has been shown. Some of these may contain many ingredients, including several pigments as well as being resin-containing, but ingredients must be separately packaged and mixed just prior to application, as has been disclosed in U.S. Pat. No. 3,297,616.
A variety of resinous and resin-free chromium containing coatings for protecting ferrous substrates are also known. Representative coating compositions can be relatively simplistic such as the compositions that may essentially contain chromic acid and particulate metal in an alcohol medium as disclosed in U.S. Pat. No. 3,687,738.
Other more complex compositions as disclosed in U.S. Pat. No. 3,907,608 may contain the pulverulent metal and hexavalent chromium-providing substance plus high-boiling organic liquid. Such coatings over ferrous surfaces provide a highly desirable protection against red rust upon exposure to the surface to salt solution.
In the protection of ferrous surfaces, it is also known to apply coating composite systems consisting of a basecoat and subsequent topcoat. For example, in the protection of zinc coated steel surfaces, the zinc surface is treated with chromate and then subsequently treated with a topcoat of a silicate material, e.g., potassium and/or sodium silicate. The chromate/silicate coating offers corrosion resistance against white rusting of the zinc basecoat, as has been discussed in Japanese Patent Disclosure No.: Showa 53-125239. Or the zinc surface, which may be phosphated, can be topcoated with a silica/organic polymer/silane complex as taught in U.S. Pat. No. 4,330,446.
A further example of composite coatings involves the application of protective coatings of silicate to resin-free basecoat compositions of particulate metal and hexavalent chromium-providing substance on ferrous substrates. This composite has been disclosed in U.S. Pat. No. 4,365,003.
Because of the corrosion resistance of such coating composites, they have attracted the attention of the automotive industry for treating small metal parts, e.g., metal fasteners, nuts, bolts and interior body panels. However, the appearance of the coating composites vary, depending on the formulation of the coating, the type of basis metal substrate and the process. Moreover, many such small parts are traditionally provided with a top-coating that will give the part a smooth as well as corrosion resistant coating, plus a lustrous black appearance. It would, therefore, be desirable to achieve a corrosion resistant coating composite having a black pigmented topcoating which can be applied to a wide variety of basecoated substrates providing superior corrosion resistance and excellent black pigmented hiding power.
It would also be desirable to provide a topcoating composition which is simple and readily manufactured from commercially available components, easily applied by conventional techniques and economical to cure. Further, desirable advantages would reside in a composition showing excellent shelf stability without the need for special packaging, a composition that is environmentally desirable being water-based to eliminate problems of pollution and toxicity and a composition amenable to water cleanup.